The present invention pertains to a process for drilling brittle materials, where, by means of a drill with a drill rod, at the forward end of which a drill head is mounted, this head carrying a drill crown which rotates around the longitudinal axis of the drill rod and which is provided with grinding or cutting means. The final hole is produced by the expansion of a pilot hole, the pilot hole being produced by a drill crown which, looking at the drill rod from the forward end in the direction of the longitudinal axis, is mounted on the drill head coaxial to and in front of the drill crown which produces the final hole.
The invention also pertains to a drill for brittle materials with a drill rod which can rotate around its longitudinal axis, at the forward end of which a drill head is mounted, this head carrying a first drill crown provided with a coating of abrasive particles, the outside dimensions of which, upon rotation around the axis of rotation, are associated with a first envelope circle, the head also carrying at least one additional drill crown, which, looking at the drill rod from the forward end in the direction of the longitudinal axis, is mounted behind the first drill crown, the outside dimensions of this second drill crown, upon rotation around the longitudinal axis, being associated with an envelope circle coaxial to the first envelope circle, the diameter of the second circle being larger than that of the first.
The product brochure entitled "Forets couronne, Core drills, Bohrkronen" from DIAMANT BOART illustrates drills which have a drill rod provided with an outside thread, by means of which the drill can be held in a drilling machine. At the forward end of the drill rod there is a drill head with a drill crown. The drill crown is in the form of a disk, which is partially recessed into the drill head. The lateral cylindrical surface of the drill crown and the end facing the material to be drilled are coated with diamond. The longitudinal axis of the drill rod represents the axis of rotation. When the drill head rotates together with the drill crown, the diamond coating acts as a grinding and cutting agent on the material to be drilled. The material is removed and a hole is thus produced. As the drill crown rotates around the axis of rotation, it describes an envelope circle, with a diameter which is the same as that of the drill crown. The diameter of the envelope circle also corresponds to the diameter of the hole produced.
A process and a drill of the general type in question are known from German Registered Design DE-GM No. 1,913,317. In this document, a rock and stone drill with several step bits and a pilot drill are described. The pilot drill and the step bits, which are mounted coaxially on the same drill head, have different diameters. Looking in the direction of the drilling rod, these diameters increase from the pilot drill in approximately equal steps. This known drill is suitable for the drilling of rock and minerals.
A drilling tool with similar features is described in DE-AS No. 1,179,525. This drilling tool has a group of rotary drilling bits, the cutting surfaces of which are coated with diamonds embedded in a binder. In one embodiment, a pilot bit is provided, the outside diameter of which is smaller than the envelope circle of the rotary drilling bits, which, looking at the tip of the drill in the direction of the drill rod, are mounted coaxial to and underneath the pilot bit. The drilling tool can also have a second group of rotary drilling bits with smaller diameters. This drilling tool is suitable for the cutting type of drilling to produce blast holes in hard rock such as sandstone, quartzite, etc.
The known drills can be used to produce holes with relatively coarse inside surfaces in coarse and hard rock. When brittle materials are drilled, damage to the material is induced in the boundary zones of the hole. It has been found that, when brittle materials such as glass and ceramic are drilled, cracks are produced in the boundary zone. These cracks extend outward from the damage point into the interior of the glass for a short distance and then proceed essentially parallel to the cut surface. These elliptical, longitudinal cracks reduce the strength of the drilled material. The higher the mechanical stresses during the removal of the material, the more pronounced the processes of crack formation and propagation.
A deep drill in the form of a tube bit for drilling deep holes is known from Dubble II, 1966, p. 549, FIG. 134. This tube bit has a shaft, the forward end of which carries a drill head, which is able to rotate around the longitudinal axis of the shaft. The drill head carries a drill crown with a circular cross section; the crown is equipped with a cutting edge. Upon rotation of the drill head, the drill crown describes an envelope circle, which is the same as the diameter of the hole produced. In an area underneath the drill crown, the drill head is provided with elevations. Upon rotation round the longitudinal axis of the shaft, these elevations describe an envelope circle with a diameter which is also the same as that of the hole. The elevations thus prevent the drill from "wandering" and therefore increase the accuracy with which it is guided.
The process to be derived from the above description and the known drill are suitable for guiding the drill head during the drilling of deep holes in metals, but not in brittle materials. The impact of the rotating elevations against the inside walls of the hole produce additional cracks in the boundary zone. These bring about a further reduction in the strength of the brittle material in which the hole has been drilled.